The present invention relates generally to the acquisition of nuclear magnetic resonance data and, more particularly, to a method for rapidly acquiring spectroscopic magnetic resonance information from which a spectroscopic image may be formed.
Nuclear magnetic resonance (NMR) techniques have long been used to obtain spectroscopic information about substances, revealing the substance's chemical composition. More recently, spectroscopic imaging techniques have been developed which combine magnetic resonance imaging (MRI) techniques with NMR spectroscopic techniques. Spectroscopic imaging techniques provide a spatial image of the chemical composition.
In recent years there has been increasing interest in the study of brain metabolism using proton MR spectroscopy and spectroscopic imaging because of its noninvasive assessment of regional biochemistry. While proton spectroscopy measures metabolite levels in a single volume, proton spectroscopic imaging (HSI) measures the spatial distribution of metabolites (e.g., N-acetylaspartate (NAA), total choline, total creatine, and lactate) over a predetermined volume of interest (VOI). HSI studies of diseased brain have shown locally altered metabolite levels in chronic and acute brain infarction, multiple sclerosis, epilepsy, brain tumors, and acquired immunodeficiency syndrome.
Spectroscopic imaging inherently requires that the pulse sequence encodes spectral information in addition to spatial information. Thus, a central problem in spectroscopic imaging is the encoding of the spectral information. For example, to perform Fourier imaging, in which the spatial image is ultimately obtained by taking a Fourier transform, the pulse sequence (i.e., both RF pulses an gradient field pulses) shown in FIG. 1 may be used. During interval 1, a 90.degree. excitation pulse is applied in the presence of gradient G.sub.z applied along the z axis. This combination excites the atoms in a single slice of an object causing the atoms of that slice to begin to decay from an excited state. As the spins of the atoms decay or evolve, a reversed gradient G.sub.z is applied during interval 2 and phase encoding gradients G.sub.x and G.sub.y are applied during intervals 2 and 3. Gradients G.sub.x and G.sub.y provide an encoding according to the location of the atoms in the selected slice. During interval 4, a 180.degree. refocusing pulse is applied, after which, during interval 5, a spin echo is received and sampled. A data set is built up by applying a number of combinations of the values of G.sub.x and G.sub.y, and the spin echoes of this data set may then be transformed together through a 5 three-dimensional Fourier transform to provide a spectroscopic image of the x-y plane in which the third dimension shows the high resolution spectrum, as explained below. The spatial resolution can be increased by sampling a large number of spin echo signals of increasing magnitudes of the phase encoding gradients G.sub.x and G.sub.y.
In the example shown in FIG. 1, the spectral information is encoded directly in the spin echo, and emerges when a Fourier transform of the spin echo is taken. Each chemical compound of a specific element will have one or more characteristic resonances offset in frequency from the basic resonance of that element. Therefore, the frequencies contained in the spin echo will correspond to the compounds of the element being imaged. When a Fourier transform of the spin echo is taken, the resulting spectrum will have a peak corresponding to each compound, the amplitude of the peak reflecting the concentration of that compound. If a data set containing several spin echoes as described above is transformed, an image showing the high resolution spectrum at each location will be produced, showing the concentrations of the compounds at each spatial location.
FIG. 2 illustrates an alternative technique for encoding the spectral information, in which a selective 90.degree. excitation pulse is similarly applied in the presence of gradient G.sub.z during interval 1. Then, during interval 2, gradient G.sub.z is reversed, and during intervals 2 and 3, phase encoding gradient G.sub.y is applied. Also during intervals 2 and 3, an initial prephasing pulse gradient G.sub.x is applied in preparation for applying G.sub.x as an observation gradient during the spin echoes. During interval 4, the 180.degree. refocusing pulse is applied. During interval 5, the observation gradient G.sub.x is again applied for a time period which is centered around the center point of the sampling period of the spin echo. As shown in FIG. 2, several different values of gradient G.sub.y are applied in the Fourier method of imaging to obtain a data set to be transformed.
To encode the spectral information in the sequence of FIG. 2, the timing of the 180.degree. pulse may be changed by an increment dt, as shown in rf sequence (b). This introduces a phase error for all spins which are not resonating at a frequency equal to the detection reference frequency, and the phase error is proportional to the frequency offset, resulting in a phase encoding of the spectral information.
The above methods for obtaining spectral information each require a large number of data acquisition sequences or shots, each beginning with an excitation, typically a selective 90.degree. excitation pulse, followed by a phase encoding interval, an echo generating waveform such as a refocusing pulse, and a spin echo sampling interval. The delay between sequence or shots is typically relatively long in relation to the length of each sequence. As a result, spectroscopic magnetic resonance techniques using conventional equipment have usually been restricted to imaging small volumes or small areas.
For example, most existing HSI techniques for human brain are based on preselection of a volume of interest (VOI) within the skull in order to reduce the undesirable resonances of water and lipid originating from areas outside the VOI. This preselection is generally achieved by a double spin echo technique or a stimulated echo technique. One or two dimensions of gradient phase encoding are employed to spatially discriminate within the VOI. Most recently, the HSI experiment has been extended to three dimensions of phase encoding, allowing the VOI to extend in all three dimensions and thereby obtaining metabolic information from a larger brain volume. Due to the large number of phase encoding steps in this experiment, the clinical limitation on the total measurement time becomes a severe restriction. For example, an experiment with 16.times.16.times.12 phase encoding steps and a repetition time (TR) of 2 seconds would require 1.5 hour of total measurement time. In the case of severely ill or instable patients, such study lengths are prohibitive. Therefore, it would be advantageous if more than one spin echo could be sampled for spectral information in each data acquisition sequence.
Techniques used in other areas of NMR gather data from a series of echoes. For instance, multiple spin-echoes per excitation pulse have been used in conventional imaging. Also, echo-planar techniques employ a pulse sequence which samples the entire k-space by a series of gradient reversals following only one excitation pulse; however, this method requires equipment modification to permit rapid gradient reversals so that all the samples are acquired within the transverse relaxation time. (Stehling et al., Science, 250, 53-60, (1990)). P. Mansfield, "Spatial Mapping of the Chemical Shift in NMR", J. of Physics D: Applied Physics, vol. 16 (1983), pp. L235-L238, discusses the application of echo-planar NMR imaging methods to the mapping of chemical shift data spectra. This technique, as noted above, requires equipment modification in order to rapidly reverse the gradient field.
It would be advantageous, however, to have a technique for acquiring NMR information including both spatial and spectral information from a series of echoes without the need for repeated reversal of a magnetic field gradient, which is relatively difficult to achieve in practice. It would also be advantageous to have such a technique which could obtain greater spatial and spectral resolution. Further, since the spectral bandwidth for chemical information is typically only 3-8 ppm, spectroscopic techniques are very sensitive to the deleterious residual effects of switched fields. Moreover, in known spin-echo methods the readout gradient typically has a short duration, thereby limiting spectral resolution. Thus, it would be advantageous to have a spectral encoding technique which does not require a readout gradient.
U.S. Pat. No. 4,628,262 to Maudsley, which is herein incorporated by reference, discloses a method for acquiring a data set for generating a spectroscopic image by generating multiple spin-echoes per excitation pulse by using a series of 180.degree. refocussing pulses. After slice selection, a series of refocussing pulses, each followed by a readout gradient (i.e., G.sub.x), is provided with the timing between the refocussing pulse and the readout gradient controlled to produce spectral encoding. Each combination of a refocussing pulse and a readout gradient induces a spin echo signal, and the readout gradient controls the time of occurrence of spin echo signal. Spectral encoding is achieved by displacing the time between at least one refocussing pulse and the readout gradient, thereby phase encoding the spectral information. Evidently, the pulse sequence which generates the spin echo signal also encodes the spectral information. For a given observed bandwidth, the delay interval, dt, must satisfy the Nyquist sampling theorem, and increased spectral resolution is achieved by increasing the number of spin echoes for a given phase encoding gradient G.sub.y.
There remains, however, a need for further improvements in spectroscopic NMR techniques.